A racetrack memory device stores information in the form of magnetic domain walls in magnetic nanowires or “racetracks”. The domain walls are moved backward and forward along the nanowires using short (˜nanosecond long) current pulses, in which the electrons carrying the current are spin polarized. The spin polarized current carries spin angular momentum, which is transferred to the domain walls causing them to move in the direction of the electron flow. For example, a series of domain walls, representing zeros and ones, may be moved to and fro along the racetrack past a reading device and a writing device, which are positioned in the middle of the racetrack. In the prior art, a reservoir is maintained in the racetrack, so that a series of domain walls can be moved across the reading and writing devices. This then allows the reading of any domain wall or the writing of a new domain wall or domain walls somewhere within the train of existing domain walls, without otherwise damaging or upsetting the series of domain walls. When only one reading and writing device is used per racetrack, the reservoir would generally need to be at least as long as the train of stored domain walls.
Thus, the prior-art racetrack memory technology favors long racetracks. To move the domain walls along the racetrack, a certain current density is required for a certain length of time. The faster the domain walls are moved, the greater the current density that is needed. However, the higher the current density is, the higher the magnitude of voltage along the racetrack that is required. The longer the racetrack, the higher is the resistance of the racetrack and the higher is the required voltage for the same magnitude of current density.
In addition, prior-art racetrack memory devices employ bi-directional motion of domain walls along the magnetic racetracks, so that domain walls can be moved in either direction across the reading and writing devices. For this purpose, electronic circuits deliver current pulses of either positive or negative polarity along the racetracks. Such racetracks are preferably designed so that the domain walls can be moved reliably in either direction along the racetrack.